1. Field of the Invention
The present invention relates to copper alloys and specifically to such alloys which exhibit high strength, high softening temperatures and excellent conductivity compared to unalloyed copper.
The ability of copper to retain its strength following exposure to elevated temperatures (termed "thermal stability" herein) is an important property for many applications in which metals are used, such as rotor and stator windings, welding electrodes, heat sinks for removal of heat from electronic devices, and articles which must be assembled by soldering. Pure copper, while having exceptional conductivity, has a tendency to experience recovery, recrystallization and grain growth at elevated temperatures as low as about 150.degree. C. which makes the pure metal unsatisfactory for many special and critical applications.
It is a well-known expedient to add various alloying elements to copper to strengthen it, but the added elements often have the undesirable effect of reducing the conductivity compared to pure copper. Alloys of copper with silver are known which exhibit desirable conductivity and good retention of strength at moderately elevated temperatures, but the high cost of the silver used to make these alloys is a drawback which limits their wider use. Thus, there is a need for copper-base compositions which exhibit higher thermal stability after exposure to elevated temperatures than copper, while exhibiting other desirable properties of copper.
2. Description of the Prior Art
While the prior art reveals that manganese and/or selenium have in the past been added to copper, there is no recognition of the very beneficial effects of adding to copper minor amounts of both manganese and selenium. For instance, U.S. Pat. No. 2,038,136 discloses adding from 0.05% to 4% selenium to copper to increase the machinability of the copper, and also discloses that the selenium-copper alloy may contain up to 0.5% manganese as an optional additive. It should be noted that the manganese and selenium contents required to improve the machinability of copper are far greater than those required by the present invention in order to improve the thermal stability of copper.
U.S. Pat. No. 4,059,437 discloses an oxygen-free copper product produced without the use of deoxidizers and containing manganese in amounts on the order of 1 to 100 ppm. The manganese is said to provide exhanced grain size control during annealing of the copper, resulting in the copper product having improved surface appearance, grain structure, and ductility after annealing, while retaining high conductivity. Other elements are disclosed as being present only in the amounts in which they normally exist in oxygen-free copper; thus, there is no suggestion of the surprisingly advantageous results of thermal stability that can be realized by incorporating both manganese and selenium into oxygen-free copper in the amounts disclosed herein.
U.S. Pat. No. 2,206,109 discloses an alloy of copper with cobalt and/or nickel, and also containing 4 to 15% manganese and up to 0.6% selenium. While this disclosure attributes improved cold workability and corrosion resistance to the manganese and selenium additives, it does not suggest a copper base alloy containing only minor amounts of manganese and selenium, and does not suggest that such an alloy would exhibit the improved properties of the present invention.
Other patents disclose adding either manganese or selenium, plus one or more other additives, to copper but fail to recognize the synergistic effect of adding both manganese and selenium in amounts within the ranges that are disclosed and claimed herein: U.S. Pat. No. 1,896,193, U.S. Pat. No. 2,178,508, U.S. Pat. No. 2,232,960, and U.S. Pat. No. 3,451,808.